feat(bot): interactive Gitea webhook bot (FastAPI) — @mention chat + auto PR review

A small persistent service (bot/app.py) that reuses the ci-ai modules
(llm.py->Jetson, gitea_api.py, ai_review prompt). Handles issue_comment
@ci-ai mentions as threaded chat and pull_request events as idempotent
reviews — answering in seconds via webhook, with NO runner/clone/hairpin.
Runs on the Pi next to Gitea (internal http://gitea:3000 API).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/gitea_api.py

@@ -86,3 +86,14 @@ def upsert_comment(owner: str, repo: str, index: int, marker: str, body: str) ->
         data={"body": full},
     )
     return created["id"]
+
+
+def post_comment(owner: str, repo: str, index: int, body: str) -> int:
+    """Legt einen NEUEN Kommentar an (nicht idempotent — für Chat-Antworten,
+    wo jede @mention ihre eigene Antwort bekommen soll)."""
+    created = _request(
+        "POST",
+        f"/repos/{owner}/{repo}/issues/{index}/comments",
+        data={"body": body},
+    )
+    return created["id"]